Fitz--Muscle Receptors

Question 1

Group Ia afferents

Answer 1

• (also called primary afferents)
• innervate all 3 types of intrafusal fibers
• provide information about both length and velocity.

Question 2

golgi tendon organs

Answer 2

• a specialized receptor that is located between the muscle and the tendon
• signals information about the load or force being applied to the muscle

Question 3

stretch reflex (myotatic reflex)

Answer 3

muscle stretch –> activation of muscle spindle Ia afferent –> enter spinal chord via dorsal root ganglion (sensory) –> synapse on alpha-MN in ventral horn (motor) –> muscle contraction

monosynaptic activation

Question 4

Group II afferents

Answer 4

• (also called secondary afferents)
• innervate the ends of the nuclear chain fibers and the static nuclear bag fibers at specialized junctions termed flower spray endings.
• Do not innervate the dynamic nuclear bag fibers,
• signal information about muscle length only.

Question 5

gamma motor neuron

Answer 5

** innervate intrafusal fibers**, which contract only slightly

gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch

Question 6

what two muscle components work together to maintain tone?

Answer 6

spindle afferents and golgi tendon organs

Question 7

flaccid paralysis

Answer 7

elimination of alpha motoneurons

Question 8

spastic paralysis

Answer 8

overactive gamma motoneurons

Question 9

reflex arc mechanism

Answer 9

1. sensory receptor
2. primary afferent neuron
3. 1-3 synapses
4. motor neuron
5. muscles

Question 10

withdrawal reflex

Answer 10

noxious stimulus –> withdrawal of limb –> contraction of flexors and relaxation of extensors

Question 11

In spinal chord lesions reflexes…

Answer 11

are functional below lesion

initially –> spinal shock,

eventually –> reflexes return (simplist first)

Question 12

Alpha motor neurons

Answer 12

• (also called lower motor neurons) innervate extrafusal fibers (skeletal muscle) and cause the muscle contractions that generate movement.
• release acetylcholine at neuromuscular junction
• anterior horn of the spinal cord and motor nuclei of the brainstem
• only neuron through which the motor system can communicate with the muscles

Question 13

How does a motor neuron control the amount of force that is exerted by muscle fibers?

Answer 13

rate code

increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force

Question 14

Muscle Spindles

Answer 14

specialized receptors that signal

(a) the length and
(b) the rate of change of length (velocity) of the muscle

Question 15

intrafusal fibers

Answer 15

are muscle spindles

extrafusal fibers are the large majority of muscle fibers that allow the muscle to do work

Question 16

There are 3 types of muscle spindle fibers:

Nuclear Chain fibers

Static Nuclear Bag fibers

Dynamic Nuclear Bag fibers

Answer 16

Nuclear Chain fibers. Nuclei are aligned in a single row (chain) in the center of the fiber. Signal information about the static length of the muscle.

Static Nuclear Bag fibers. Nuclei are collected in a bundle in the middle of the fiber. Like the nuclear chain fiber, these fibers signal information about the static length of a muscle.

Dynamic Nuclear Bag fibers. Anatomically similar to the static nuclear bag fibers, but they signal primarily information about the rate of change (velocity) of muscle length.

Question 17

inverse stretch reflex, where strong contraction is followed by muscle relaxation…

Answer 17

• due to GTO afferents (1b) activation of an inhibitory interneuron
• stretch –> depolarization –> increased inhibitory interneuron –> decrease alpha motor neuron activity –> relaxation

Question 18

In summary,

muscle spindle vs. golgi tendon organ

Answer 18

• Muscle spindles signal information about the length and velocity of a muscle
• Golgi tendon organs signal information about the load or force applied to a muscle

Question 19

Reciprocal inhibition in stretch reflex

Answer 19

mechanism through which the opposing muscle group is inhibited

accomplished by an inhibitory interneuron in the spinal cord

Ia afferent of the muscle spindle bifurcates –> Ia inhibitory interneuron –> inhibits alpha-MN to opposing muscle (relaxation)

Question 20

Golgi tendon organ is involved in a spinal reflex known as the autogenic inhibition reflex. When tension is applied to a muscle, how does this work?

Answer 20

Afferent Ib fibers activated –> sensory (DRG) to dorsal column –> synapses onto Ib inhibitory interneuron –> inhibits alpha motor neuron that innervates the same muscle that caused the GTO Ib afferent to fire.

Ib fibers (sensory) also bifurcate, and excite an interneuron to alpha-MNs of the opposing muscle.

Remember that GTOs respond to force!

Question 21

Flexor reflex (withdrawal reflex)

Answer 21

Pain –> Group III afferents (sensory/DRG) –> excitatory interneuron –> activation of alpha-MNs to flexor muscles (up and down spinal cord)

Note that Group III afferents also activate inhibitory interneurons to extensor alpha-MNs.